They work like a charm.. Its not just "light" its UVC radiation being emitted from the bulb. A portion of this energy shows up as light, but the rest of it is a toxic radiation dose that destroys the cellular membrane of algae and critters causing it to die. The dead material is then removed by mechanical filtration (your existing filters)
When you are sizing a UV sterilizer for your tank you need to check the flow rating on the UV sterilizer.. they should have a sizing chart so you can choose the right model for your sized Tank.
I use 18 watt Sterilizer onmy 135 gallon